As recently as five years ago most chemists had never heard of ionic liquids (IL). But since then interest in these unorthodox materials has grown at a phenomenal rate. R. D. Rogers and K. R. Seddon, in Ionic Liquids as Green Solvents-Progress and Prospects, ed. By R. D. Rogers and K. R. Seddon, ACS Symp. Ser. 856, ACS, Washington, D.C., 2003, p. xiii. The scope of demonstrated or proposed applications of IL is extraordinary, ranging from their use as non-volatile, non-flammable solvents to advanced heat transfer fluids, lubricants and anti-statics. M. Freemantle Chem. Eng. News 2004, 82(45), 44; and M. Freemantle Chem. Eng. News 2004, 82(18), 26. Surpassing in magnitude the number of potential uses is the number of possible IL compositions, estimated by Seddon to be in the billions. K. R. Seddon, in The International George Papatheodorou Symposium: Proceedings, S. Boghosian, V. Dracopoulos, C. G. Kontoyannis and G. A. Voyiatzis, Eds.; Institute of Chemical Engineering and High Temperature Chemical Processes: Patras, 1999; p 131.
Ionic Liquids. Ionic liquids consist of ions. However, unlike conventional molten salts (for example, molten sodium chloride), ionic liquids often melt below 100° C. When an ionic liquid has a melting point below room temperature, it is said to be a room-temperature ionic liquid. Since their melting points are low, room-temperature ionic liquids can act as reaction solvents. Because an ionic liquid is made of ions rather than molecules, they often provide distinct selectivities and reactivities as compared to conventional organic solvents.
Room-temperature ionic liquids have been used as clean solvents and catalysts for green chemistry and as electrolytes for batteries, photochemistry and electro-synthesis. They have no significant vapor pressure and thus create no volatile organic contaminants. They also allow for easy separation of organic molecules by direct distillation without loss of the ionic liquid. Their liquid range can be as large as 300° C. allowing for large reaction kinetic control, which, coupled with their good solvent properties, allows small reactor volumes to be used. Salts based upon poor nucleophilic anions, such as [BF4]−, [PF6]−, [CF3CO2]−, and [CF3SO3]−, are water and air insensitive and possess remarkably high thermal stability. Many of these materials are based around an imidazolium cation, 1-alkyl-3-methylimidazolium. By changing the anion or the alkyl chain on the cation, a wide variation in properties, such as hydrophobicity, viscosity, density and solvation, can be obtained. For example, ionic liquids will dissolve a wide range of organic molecules to an appreciable extent, the solubility being influenced by the nature of the counter anion.
The unique physical properties of ionic liquids have been found to offer certain advantages in numerous applications. For example, U.S. Pat. No. 5,827,602 to Koch et al. discloses ionic liquids having improved properties for application in batteries, electrochemical capacitors, catalysis, chemical separations, and other uses. The ionic liquids described in Koch et al. are hydrophobic in nature, being poorly soluble in water, and contain only non-Lewis acid anions. When fluorinated, they were found to be particularly useful as hydraulic fluids and inert liquid diluents for highly reactive chemicals. In addition, ionic liquids have been discussed by Freemantle, M. Chem. Eng. News 1998, 76 [March 30], 32; Carmichael, H. Chem. Britain 2000, [January], 36; Seddon, K. R. J. Chem. Tech. Biotechnol. 1997, 68, 351; Welton, T. Chem. Rev. 1999, 99, 2071; Bruce, D. W., Bowlas, C. J., Seddon, K. R. Chem. Comm. 1996, 1625; Merrigan, T. L., Bates, E. D., Dorman, S. C., Davis, J. H. Chem. Comm. 2000, 2051; Freemantle, M. Chem. Eng. News 2000, 78 [May 15], 37; Holbrey, J. D., Seddon, K. R. Clean Products and Processes 1999, 1, 223-236; and Dupont, J., Consorti, C. S. Spencer, J. J. Braz. Chem. Soc. 2000, 11, 337-344.
Ionic liquids have been used as solvents for a broad spectrum of chemical processes. These ionic liquids, which in some cases serve as both catalyst and solvent, are attracting increasing interest from industry because they promise significant environmental benefits, e.g., because they are nonvolatile they do not emit vapors. Hence, for example, they have been used in butene dimerization processes. WO 95/21871, WO 95/21872 and WO 95/21806 relate to ionic liquids and their use to catalyze hydrocarbon conversion reactions, such as polymerization and alkylation reactions. The ionic liquids described for this process were preferably 1-(C1-C4alkyl)-3-(C6-C30 alkyl) imidazolium chlorides and especially 1-methyl-3-C10 alkyl-imidazolium chloride, or 1-hydrocarbyl pyridinium halides, where the hydrocarbyl group is, for example, ethyl, butyl or other alkyl. PCT publication WO 01/25326 to Lamanna et al. discloses an antistatic composition comprising at least one ionic salt consisting of a nonpolymeric nitrogen onium cation and a weakly coordinating fluoroorganic anion, the conjugate acid of the anion being a superacid, in combination with thermoplastic polymer. The composition was found to exhibit good antistatic performance over a wide range of humidity levels.
However, it has been pointed out that touting the environmental benefits of IL chemistry is something that should be done with care. J. D. Holbrey, M. B. Turner and R. D. Rogers in Ionic Liquids as Green Solvents-Progress and Prospects; R. D. Rogers and K. R. Seddon, Eds.; ACS Symposium Series 856; American Chemical Society: Washington, D.C. 2003; 2. In a recent paper, a commentary has been offered on this situation as it pertains to fluorous anions, which are the most widely used anion type in IL formulations. R. P. Swatlowski, J. D. Holbrey and R. D. Rogers Green Chem. 2003, 5, 361. While there are situations in which IL with fluorous anions will remain indispensable, there is much to be desired in identifying other (preferably innocuous) ions in formulating IL, especially for large-volume applications. J. H. Davis, Jr. and P. A. Fox Chem. Commun. 2003, 1209; R. P. Swatlowski, J. D. Holbrey and R. D. Rogers Green Chem. 2003, 5, 361. To this end, non-toxic organoanions such as acetate and lactate have been used to formulate IL. M. J. Earle, P. B. McCormac and K. R. Seddon, Green Chem. 1999, 1, 23. However, carboxylates are basic, readily engage in hydrogen bonding, and are strongly coordinating towards transition-metal ions. Such attributes are not typical of the fluorous anions on which so many IL compositions are based. Mapped onto an IL, these properties are likely to be useful in some circumstances and detrimental in others.
Brönsted Acid Catalysis. From undergraduate laboratories to chemical manufacturing plants, the use of strong Brönsted acids is ubiquitous. Smith, M. B.; March, J. March's Advanced Organic Chemistry; Wiley-Interscience: New York, 2001; Chapter 8. In this context, solid acids are being more widely used since, as non-volatile materials, they are deemed less noxious than traditional liquid acids. Ritter, S. K Chem. Eng. News 2001, 79 (40), 63-67. However, solid acids have shortcomings. Among the more troublesome of these are restricted accessibility of the matrix-bound acidic sites, high mw/active site ratios, and rapid deactivation from coking. Ishihara, K.; Hasegama, A. and Yamamoto, H. Angew. Chem. Int. Ed. 2001, 40, 4077-4079; and Harmer, M. A. and Sun, Q. Appl. Catal. A: General 2001, 221, 45-62.
Bearing in mind both the advantages and disadvantages of solid acids, the search continues for systems that are Brönsted acids with solid-like non-volatility but which manifest the motility, greater effective surface area and potential activity of a liquid phase. Combining just these characteristics, ionic liquids have been described as one of the most promising new reaction mediums. Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68, 351-356. Not only can these unusual materials dissolve many organic and inorganic substrates, they are also readily recycled and are tunable to specific chemical tasks. Bates, E. D.; Mayton, R. D.; Ntai, I. and Davis, J. H. Jr. J. Am. Chem Soc. 2002, 124, 926-927; Visser, A. E.; Holbrey, J. D.; Rogers, R. D. Chem. Commun., 2001, 2484-2485; Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Mayton, R.; Sheff, S.; Wierzbicki, A.; Davis, J. H. Jr.; Rogers. R. D. Chem. Commun., 2001, 135-136; Merrigan, T. L.; Bates, E. D.; Dorman; S. C.; Davis, J. H. Jr. Chem. Commun. 2000, 2051-2052; Forrester, K. J.; Davis, J. H. Jr. Tetrahedron Letters, 1999, 40, 1621-1622; and Morrison, D. W.; Forbes D. C.; Davis, J. H. Jr. Tetrahedron Letters, 2001, 42, 6053-6057.
Further, the chemical industry is under significant pressure to replace the volatile organic compounds that are currently used as solvents in organic synthesis. Many of these solvents, such as chlorinated hydrocarbons, are toxic and hazardous for the environment, due to their emissions in the atmosphere and the contamination of aqueous effluents. Ionic liquids seem to offer a solution to this problem, too. Ionic liquids have no measurable vapor pressure. This means that they don't evaporate, and therefore they emit no hazardous vapors in the atmosphere, and replenishing of the solvent is not required. This property also allows easy separation of volatile products. Ionic liquids are able to dissolve a wide range of organic, inorganic and organometallic compounds. Notably, their properties can be adjusted by altering the cation or anion of the IL, allowing for fine tuning of the reaction.
Moreover, many organic transformations, such as Fischer esterification, alcohol dehydrodimerization and the pinacol/benzopinacol rearrangement, require an acidic catalyst. Solid acids are now being used, because, as nonvolatile compounds, they are less hazardous than traditional liquid acids. As noted above, although they are less hazardous, solid acids have several disadvantages, such as restricted accessibility of the matrix-bound acidic sites, high molecular weight/active-site ratios, and rapid deactivation from coking. Cole, A. C.; Jensen, J. L.; Ntai, I.; Tran, K. L. T.; Weaver, K. J.; Forbes, D. C.; Davis, J. H., Jr. J. Am. Chem. Soc. 2002, 124, 5962-5963.
Purification of Gas Mixtures. There is little doubt that petroleum, coal and natural gas will continue to be the primary global fuel and chemical feedstock sources for some years to come. Natural gas is regarded as the cleanest of these materials, and as such is being consumed at an accelerating pace. Despite its reputation as a clean fuel, natural gas is usually contaminated with a variety of undesirable materials, especially CO2 and H2S. While this level of contamination is very low in gas from certain sources (sweet gas), it is much higher in gas from others (sour gas). As sweet gas reserves are depleted, pressures will build for the increased utilization of sour gas. Oil and Gas R&D Programs: Securing the U.S. Energy, Environmental and Economic Future. Office of Fossil Energy, U.S. Dept. of Energy, Office of Natural Gas and Petroleum Technology: Washington, D.C., 1997. Since admixed CO2 lowers the fuel value of natural gas, the large amount of it present in sour gas compels its removal prior to combustion. The lower fuel value for sour gas, coupled with the connection between CO2 and global warming, makes CO2 capture a commercially important and environmentally desirable process.
One of the most attractive approaches for the separation of a target compound from a mixture of gases in a gas stream is selective absorption into a liquid. Astarita, G,; Savage, D. W.; Bisio, A. Gas Treating with Chemical Solvents; Wiley-Interscience: New York, 1983. Such interactions between gases and pure liquids or solutions are the bases for numerous gas separation technologies, including commercial systems for the removal of CO2 from natural gas. These scrubbing processes include ones in which the simple, differential dissolution of the target gas into the liquid phase is of principal importance. More common are processes in which a chemical reaction of the target gas with a solute in the liquid phase is the main mode of sequestration. With either mode of gas removal, the vapor pressure of the solvent itself plays a significant role in gas-liquid processes, usually to their detriment. In the case of large-scale CO2 capture, aqueous amines are used to trap chemically the CO2 by way of ammonium carbamate formation. In these systems, the uptake of water into the gas stream is particularly problematic. Compounding the water uptake difficulty is the loss into the gas stream of the volatile amine sequestering agent.
A liquid that could facilitate the sequestration of gases without concurrent loss of the capture agent or solvent into the gas stream should prove to be a superior material in such applications. To this end, ionic liquids (low temperature molten salts) have been proposed as solvent-reagents for gas separations. Pez, G. P. et al. U.S. Pat. No. 4,761,164. Due to the coulombic attraction between the ions of these liquids, they exhibit no measurable vapor pressure up to their thermal decomposition point, generally greater than 300° C. This lack of vapor pressure makes these materials highly attractive for gas processing. Indeed, for these purposes they may be thought of as “liquid solids,” incorporating some of the most useful physical properties of both phases.
Despite the general promise of ionic liquids in gas treatment, the molten salts used thus far for CO2 separation are generally “off the shelf” materials, such as (CH3)4NF tetrahydrate, that are not optimized for this purpose, frequently depending upon another volatile reagent, water. Pez, G. P. et al. U.S. Pat. Nos. 4,761,164 and 4,973,456; and Quinn, R.; Appleby, J. B.; Pez, G. P. J. Am. Chem. Soc., 1995, 117, 329. For instance, the latter salt uses the very weakly basic bifluoride ion to drive the net generation of bicarbonate from CO2 and water.
Electrolytic Solutions. An ionic compound generally forms crystals in which positively charged cations and negatively charged anions pull electrostatically against each other. When this ionic compound is dissolved in various other liquids, including water, it provides a liquid that carries electricity; that is, an electrolyte solution. Electrolyte solutions obtained by dissolving an ionic compound in an organic solvent are commonly used in, for example, nonaqueous electrolyte batteries and capacitors.
The chemical species present in the ionic liquids are all charged cations or anions; no neutral atoms or molecules are present. Therefore, elements which cannot be obtained from an aqueous electrolyte solution because they have too large a reducing or oxidizing power with respect to water, including metals such as alkali metals, aluminum and rare-earth elements, and non-metals such as fluorine, can be electrolyzed in a ionic liquid and obtained in elemental form. This has become a main industrial application of molten salts.
Research is actively being pursued on applications for such ionic liquids in electrolytic deposition and in electrolytes for batteries and other purposes. However, because ionic liquids generally have a high moisture absorption and are difficult to handle in air, such applications has yet to be fully realized. In light of these circumstances, one aspect of the invention is to provide ionic liquids which can be easily and efficiently produced; electrolyte salts for electrical storage devices which have excellent solubility in organic solvents for nonaqueous electrolyte solutions and have a low melting point; liquid electrolytes for electrical storage devices which include these electrolyte salts; and electrical double-layer capacitors and secondary batteries of excellent low-temperature properties which are constructed using such liquid electrolytes.
Future Outlook. The prospects for preparing a broad array of ionic liquids with ions incorporating functional groups are good. Moreover, certain of these new “task-specific” ionic liquids have proven useful in both synthetic and separations applications. Visser, A. E.; Holbrey, J. D.; Rogers, R. D. Chem. Commun., 2001, 2484; Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Mayton, R.; Sheff, S.; Wierzbicki, A.; Davis, J. H. Jr.; Rogers. R. D. Chem. Commun., 2001, 135; Merrigan, T. L.; Bates, E. D.; Dorman; S. C.; Davis, J. H. Jr. Chem. Commun. 2000, 2051; Fraga-Dubreuil, J.; Bazureau J. P. Tetrahedron Lett., 2001, 42, 6097; and Forrester, K. J.; Davis, J. H. Jr. Tetrahedron Lett., 1999, 40, 1621.
In the absence of predictive computational methods to direct their design, the discovery-based development of new IL will remain vital to the field. This is especially the case vis-à-vis heretofore unknown or unused classes of ions when such entities are easily prepared and provide access to potentially unique structural or electronic attributes. E. B. Carter, S. L. Culver, P. A. Fox, R. D. Goode, I. Ntai, M. D. Tickell, R. K. Traylor, N. W. Hoffman and J. H. Davis, Jr. Chem. Commun. 2004, 630. In light of these considerations we disclose herein that an obscure cation type—the “boronium” ion—is a versatile platform for creating hydrophobic, room-temperature ionic liquids.